The present invention concerns an evaporated fuel treatment apparatus for an internal combustion engine that releases evaporated fuel generated inside the fuel tank into the intake manifold of the internal combustion engine. More particularly, the present invention concerns an evaporated fuel treatment apparatus for an internal combustion engine that makes it possible to ascertain the presence or absence of leakage in an evaporated fuel discharge prevention system extending from the fuel tank to the engine intake system.
A method for judging the presence or absence of leakage in a tank system is described in Japanese Patent Application Kokai No. Hei 7-83125. In this method, the pressure in an evaporated fuel discharge prevention system is lowered to a predetermined pressure; next, with the target pressure reduction value of the pressure in the fuel tank alternately set at an upper-limit value and a lower-limit value, a feedback pressure reduction process which gradually causes the pressure in the fuel tank to converge on the target pressure reduction value is performed, and the amount of pressure shift in the fuel tank per unit time is calculated (leak down checking mode). In order to eliminate the effect of vapor on the judgement results, the amount of pressure shift per unit time caused by the evaporated fuel is calculated as a correction value. Judgement of the presence or absence of leakage in the tank system is accomplished on the basis of a value obtained by subtracting the value produced by multiplying the amount of pressure shift calculated in the correction checking mode by a coefficient from the amount of pressure shift calculated in the leak down checking mode. If this value is equal to or less than a predetermined value, it is judged that the tank system is normal, with no leakage; on the other hand, if this value exceeds the predetermined value, it is judged that there is leakage in the tank system.
However, in cases where the amount of vapor generated during the leak check described above is extremely large, the internal pressure of the fuel tank may fluctuate from a negative pressure to a positive pressure. A detailed examination of the shift patterns of the internal pressure of the fuel tank during the leak checking mode reveals that (a) the amount of pressure shift per unit time increases with increasing negative pressure, (b) the amount of pressure shift decreases as atmospheric pressure is approached, and (c) the amount of pressure shift tends to be small in the case of a positive pressure. Accordingly, in cases where the internal pressure of the fuel tank fluctuates to a positive pressure during the leak check, the overall amount of shift in the internal pressure of the fuel tank is extremely small; as a result, the system may be erroneously judged to be normal even when leakage has occurred. Furthermore, since a long period of time ranging from 30 seconds to 60 seconds is required in order to detect leakage caused by extremely small holes such as holes with a diameter of 0.5 mm, cases in which a large amount of vapor is generated during this period may easily occur.
In such cases, leakage may be detected even though no leakage actually exists, and if the warning lamp is frequently lit as a result, this can challenge the practical utility of the vehicle. Conversely, if no leakage is detected even though leakage actually exists, this has a deleterious effect on the environment.
An evaporated fuel treatment apparatus is constructed which is characterized by the fact that in an evaporated fuel treatment apparatus which has (a) a leakage checking means which detects the conditions of change in the internal pressure of the fuel tank when said fuel tank is closed after being placed under a negative pressure, and (b) a judgement means which judges the presence or absence of leakage in the fuel tank on the basis of the detection results obtained by the leakage checking means and correction checking means. The apparatus further including a pressure variation checking means which detects the degree of pressure variation in the fuel tank, and then opens the fuel tank to the atmosphere and judges whether or not the pressure inside the fuel tank has dropped, and a judgement prohibition means which prohibits a judgement of the presence or absence of leakage from being made by the judgment means, in response to a judgement by the pressure variation checking means that the pressure has dropped. Accordingly, a judgement of the presence or absence of leakage in the tank system is prevented from being performed in cases where such a judgement might be inaccurate.
The fuel tank is opened to the atmosphere following the tank leak check, and if the internal pressure of the fuel tank is found to have fluctuated from a positive pressure to atmospheric pressure, this means that an accurate judgement of the presence or absence of leakage in the tank system cannot be made. Accordingly, such a judgement is prohibited. As a result, judgments can be avoided under conditions in which an erroneous judgement might occur in the judgement of the presence or absence of leakage in the tank system, so that the reliability of such judgments can be improved.